1. Field of the Invention
The present invention relates to a television-telephone apparatus capable of conducting communication both in voice and image and, more particularly, to a television-telephone apparatus having a message keeping function.
2. Description of the Related Art
In recent years, digital public communication networks, generally referred to as ISDNs (Integrated Services Digital Networks), become widespread thereby enabling simultaneous telecommunications both in image and voice. Consequently, services such as television-telephone services and television-assisted meeting systems have made forward steps towards practical use. Nowadays, various rules concerning voice/image services using digital networks, protocols and multi-media multiplex frame structures have been proposed as recommendations H.320, H.242, H.221 and so forth, by CCITT (Committee Consulting International Telegraph and Telephone).
Recommendation H.320 determines general system aspects of voice and image services, while H.221 determines encoding allocations of FAS (Frame Alignment Signal) and BAS (Bit-rate Allocation Signal) which are used in appointment of exchange and communication modes of the terminal capacity, as well as frame structure, in voice/image transmission on 64 Kbps-1,920 Kbps channel. H.242 governs the protocol for capacity exchange and communication mode change-over between terminals performed by BAS.
The recommendations also describe setting end-to-end physical connections, as well as a method for multi-communication between terminals for sending and receiving image, voice and other data in accordance with an in-channel terminal capacity exchange sequence by BAS or in accordance with mode change-over sequence under appointment of communication mode, after in-channel establishment of synchronization by FAS.
Each terminal is allowed to change its terminal capacity according to conditions, and which one of the communication modes is used within the exchanged capacity is not ruled by the recommendations.
When image information, voice information and data are transmitted simultaneously, the transmission rate of the voice information is determined by the method of encoding of the voice information, while the transmission rate of the data is set to a designated value, and the remainder of the transmission capacity of the transmission rate in the communication network is allocated to the transmission of image information.
As a method of compressing image information, the recommendation H.261 of CCITT proposes an encoding method in which intra-frame encoding and inter-compensating-frame encoding are mixed.
In an ordinary telephone system for voice communication alone, a message keeping function has been conveniently used for recording a voice message in a magnetic tape or a memory card, when a call is received during one's absence. It is desirable that a similar message keeping function is available also in an image communication system.
It would be possible to restore the compressed image and record the restored image in a video tape in an analog manner. Such a recording method, however, will require a large scale recording device and, in addition, considerably impairs the quality of the reproduced image due to degradation of image quality incurred during analog recording and reproduction. Digital recording of the restored image also is impractical because of the huge amount of data.
Alternatively, storing the received compressed image information in a large-scale storage device such as a hard disk, without restoration, has been studied. This method also is impractical because the intra-frame encoded frames and inter-frame encoded frames coexist according to the recommendation H.261 of CCITT. Namely, decoding of inter-frame encoded frames essentially requires reference to the data of the preceding frame. Mere storage of the received compressed image information in a storage device cannot provide correct image information when the latter is reproduced and displayed.
More specifically, according to the recommendation H.261 of CCITT, a television telephone apparatus of this kind employs various encoding methods according to the state of the transmitted image, such as Inter (Inter-Frames Encoding), MC (Motion Compensation Encoding) and INTRA (Intra Frame Encoding). The quality of the received images has been degraded to some extent when the encoding has been done in Inter or MC mode. Therefore, when the image received in Inter or MC mode has been stored as the received image message data, the quality of the image reproduced and displayed is undesirably impaired.
A known television telephone apparatus having a message keeping function also suffers from the following problem. The apparatus in message keeping mode records the received image data as it is, as the image message to be kept. When the time available for recording is going to expire, a warning sound or a beep is given to the caller to inform that the time available for the recording is going to expire. This warning method tends to confuse the caller, particularly when the caller is not familiar with this type of apparatus, often resulting in a misunderstanding of the meaning of the warning sound.
A known television telephone apparatus also has a responding function in which a motion picture composed of a plurality of image frames is stored as a response message image and, when a call is received during one's absence, the response message image is sent back to the caller. Such responding function requires a very large memory capacity, as well as complicated hardware structure, raising production costs.